1. Field of the Invention
The present invention relates generally to a PLL control method of controlling a phase of a clock of a PLL (Phase Locked Loop) for taking synchronism of a receiving signal in a data receiving apparatus such as MODEM etc and, more particularly, to a PLL control method for obtaining a PLL control signal by making use of a tap coefficient of an auto equalizer.
2. Description of the Related Art
A MODEM (modulator-demodulator) is widely utilized in a data communications network using lines. In this type of MODEM, it is necessary for stable data communications to generate a PLL clock for taking the synchronism of a receiving signal.
FIG. 9 is a diagram showing a spectrum in a Nyquist transmission. FIG. 10 is a diagram showing a spectrum in a non-Nyquist transmission. FIG. 11 is a diagram illustrating a construction in the prior art. FIGS. 12A-12C are explanatory diagrams each showing the prior art.
According to the prior art PLL control method in the MODEM, a timing phase component is extracted from a receiving carrier in steady-state communications after an end of training, and a PLL control signal is obtained. This PLL control method is, in the case of a Nyquist transmission system, because of an existence of a Nyquist frequency, capable of easily extracting the timing phase from the receiving carrier.
Namely, as shown in the spectrum diagram of FIG. 9 illustrating a frequency distribution in the Nyquist transmission, a Nyquist transmission method is a method of transmitting at a Nyquist interval signal points disposed on at the Nyquist interval (a Nyquist frequency) on the time-axis. According to the Nyquist transmission method, the signal points can be transmitted without any inter-symbol interference with other signal points.
Incidentally, some of the MODEMs recently proposed have a transmission speed as high as, e.g., 28.8 kbps. These MODEMs adopt a non-Nyquist transmission method without using the Nyquist transmission method.
As shown in the spectrum diagram of FIG. 10 illustrating a frequency distribution in the non-Nyquist transmission, the non-Nyquist transmission method is a method of transmitting at the Nyquist interval the signal points disposed between the two Nyquist intervals on the time-axis. According to the non-Nyquist transmission method, the signal points are transmitted having the inter-symbol interference with other signal points.
In the thus structured non-Nyquist transmission method, there exists no Nyquist frequency, and hence it is difficult to extract the timing component.
Further, a phase delay distortion might occur in the signal because of being transmitted via a carrier link depending on the line through which the signal is transmitted. it is therefore difficult to extract a timing phase.
Thus, in the case of using the non-Nyquist transmission method and in the steady state, the extraction of the timing component from the receiving carrier becomes difficult depending on the state of the line. Consequently, the PLL becomes unstable, and there might be induced a deterioration of performance such as an increase in error rate of the MODEM.
There is proposed a method of obtaining a stable PLL control signal without using the above timing extracting method in the steady state (Japanese Patent Application Laid-Open Publication No.8-335962). This method is explained with reference to FIGS. 11 and 12A through 12C.
FIG. 11 is a diagram showing a construction of a receiving unit of a MODEM. As shown in FIG. 11, an analog signal (a receiving signal) received via the line is converted by an analog/digital converter 90 into a digital signal. The digital signal outputted from the A/D converter 90 is demodulated by a demodulation unit 91.
An output from the demodulation unit 91 is waveform-shaped by a roll-off filter (ROF) 92. An output of the roll-off filter 92 is subjected to auto gain control in an auto gain control unit (AGC) 93 and is thereafter waveform-shaped by an auto equalization unit (EQL) 94. Then, an output of the auto equalization is inputted to an unillustrated judging unit, whereby receiving data is regenerated.
On the other hand, on the occasion of transmitting the data, before transmitting the data, training data having a predetermined pattern is to be transmitted. A receiving apparatus demodulates this item of training data, and executes an initializing process of each unit. Herein, a timing extraction/phase rotational quantity calculating unit 95 extracts the timing component in the training signal transmitted anterior to the data signal, and calculates a phase rotational quantity of the timing component.
A selection unit 97 selects this phase rotational quantity as a PLL control signal during the training period. With this selection, during the training period, the timing PLL unit (TIM PLL) 98 is phase-controlled by the timing phase of the training signal.
Namely, the timing PLL unit 98 controls a phase of an internal clock so that the phase rotational quantity is zeroed. This phase-controlled sampling signal (the clock) is outputted to the A/D converter 90. The phase of the sampling signal is thereby synchronized with the phase of the training signal.
During data communications (a steady state) subsequent to the training period, as described above, it is difficult to extract a timing phase component from a carrier of the receiving signal. Therefore, the PLL control signal is obtained from a tap coefficient of the auto equalization unit 94. The auto equalization unit 94 is constructed of a transversal filter to eliminate a distortion of waveform of the receiving signal due to a line distortion etc.
The auto equalization unit 94 dynamically controls the tap coefficient so as to obtain an optimal equalization characteristic suited to a line state. During the training period, the tap coefficient is initialized based on the training data. Corresponding to the line state, the tap coefficient of a center tap located at the center of a group of taps of the auto equalization unit 94, grows most, and the tap coefficients of the taps adjacent on both sides to the center tap grow to some extent corresponding to the line state. The optimal equalization characteristic at that point of time is thereby attained.
Herein, if the timing phase is synchronized therewith, the tap coefficient holds the same value as a value immediately after the training. Whereas if the timing phase is shifted, the tap coefficient set at the training time moves correspondingly in the right or left direction on the basis of the center tap.
FIG. 12A shows a distribution of the tap coefficients of the respective taps when the timing phase is synchronized. In this case, the center tap value is largest, and the right and left tap coefficients are set to values corresponding to the line state.
FIG. 12B shows a distribution of the tap coefficients of the taps when the timing phase lags. In this case, for compensating the phase lag, the tap coefficients are controlled so that the tap coefficient of the tap disposed on the right side of the center tap takes a large value, while the tap coefficient of the tap disposed on the left side of the center tap takes a small value.
FIG. 12C shows a distribution of the tap coefficients of the taps when the timing phase advances. For compensating the phase advancement, the tap coefficients are controlled so that the tap coefficient of the tap disposed on the left side of the center tap takes a large value, while the tap coefficient of the tap disposed on the right side of the center tap takes a small value.
Accordingly, the phase advancement/lag of the receiving signal can be detected corresponding to the values of the tap coefficients. Since the tap coefficients are properly set at the training time, the PLL control is conducted so that the distribution of the tap coefficients is held during the steady state as it is at the training time. Therefore, according to the prior art, there is provided a tap right-and-left difference extraction unit 96 (see FIG. 11) for taking a difference between a sum of the tap coefficients of the taps on the left side of the center tap and a sum of the tap coefficients of the taps on the right side thereof. Then, during the steady state, the selection unit 97 selects an output of the tap right-and-left difference extraction unit 96 as a PLL control input, and the timing PLL unit 98 is controlled so that the above difference is zeroed.
There arise, however, the problems inherent in the prior art.
Since there is a scatter in the line characteristic, depending on the line characteristic, the output of the tap right-and-left difference extraction unit does not become zero as the case may be immediately after training for the training. For the line exhibiting such a line characteristic, the PLL control is executed so that the tap right-and-left difference extraction output is zeroed. Hence, there exists a problem, wherein the phase is transiently shifted untill the tap right-and-left difference extraction output is zeroed immediately after training, and an unstable state occurs. The data transmission just after being led in has been unstable.
Further, even if the tap right-and-left difference extraction output does not become zero immediately after training, the PLL control is implemented so that tap right-and-left difference extraction output is zeroed. This might conduce to such a problem of being synchronized with a phase different from the timing phase when training, it is therefore difficult to attain the precise phase synchronization.
It is a primary object of the present invention to provide a PLL control method in a data receiving apparatus, of taking synchronism with a phase when training irrespectively of a line characteristic.
It is another object of the present invention to provide a PLL control method in a data receiving apparatus, of preventing an occurrence of a transiently-shifted state immediately after training irrespective of a line characteristic.
It is still another object of the present invention to provide a PLL control method in a data receiving apparatus, of attaining precise phase synchronization even by using tap coefficients irrespective of a line characteristic.
To accomplish the above objects, according to a first aspect of the present invention, a PLL control method in a data receiving apparatus including an auto equalization unit for automatically equalizing a demodulated receiving signal, comprises a step of training a timing phase of PLL in accordance with a training signal, a step of obtaining, immediately after training, a right-side reference value from a sum of tap coefficients of taps on the right side of a center tap of the auto equalization unit, and obtaining a left-side reference value from a sum of tap coefficients of taps on the left side of the center tap of the auto equalization unit, a step of calculating, after obtaining the two reference values, a first sum of the tap coefficients of the taps on the right side of the center tap of the auto equalization unit, thereafter calculating a first difference between the first sum and the right-side reference value, calculating a second sum of the tap coefficients of the taps on the left side of the center tap of the auto equalization unit, and thereafter obtaining a second difference between the second sum and the left-side reference value, and a step of obtaining a PLL control signal by obtaining a difference between the first and second differences.
According to the present invention, the sum of the right-side tap coefficients and the sum of the left-side tap coefficients just after training, are stored as the right- and left-side reference values. Thereafter, the sum of the right-side tap coefficients is calculated, then the first difference between this sum and the right-side reference value is calculated. Then, the sum of the left-side tap coefficients is calculated, and the second difference between this sum and the left-side reference value. A deviation from the phase (the reference value) immediately after timing-training can be thereby known. Then, a difference between the first and second differences is taken, thereby obtaining the PLL control signal. Therefore, a shift quantity and a shift direction are thereby obtained.
Thus, according to the present invention, the sum of the right-side tap coefficients and the sum of the leftside tap coefficients just after training, are stored as the right- and left-side reference values, and the PLL control is effected so that the difference from the reference value becomes zero. Hence, even if there is the scatter in the line characteristic, the phase can be adjusted precisely to the phase when training. Further, there is executed the PLL control to the reference value immediately after training, and it is therefore feasible to prevent an occurrence of a transiently shifted state just after training, and the stable data transmission can be carried out immediately after training.
According to a second aspect of the present invention, the step of executing the training process includes a step of extracting a timing component of the training signal, and thereafter calculating a phase rotational quantity of the timing component.
According to a third aspect of the present invention, the step of obtaining the reference values includes a step of storing the obtained right- and left-side reference values.
According to a fourth aspect of the present invention, the PLL control method further comprises a step of determining, after training, the center tap of the auto equalization unit in accordance with values of the tap coefficients of the taps of the auto equalization unit.
According to a fifth aspect of the present invention, the PLL control method further comprises a step of obtaining, after training, a timing reference value from a phase of the timing component regenerated from the receiving signal, a step of obtaining a third difference between an average value of the phase of the timing component regenerated from the receiving signal and the reference value, and a step of obtaining the PLL control signal by adding the third difference to the difference between the first and second differences.
Other features and advantages of the present invention will become readily apparent from the following description taken in conjunction with the accompanying drawings.